To dry the printing or to polymerize the ink or coating on a metal sheet, particularly a tin plate sheet, preparatory to making it into cans, ultraviolet lamps may be used. It is desirable continuously to monitor the ultraviolet lamps to determine if they are bright enough to perform properly. To that end, one may place ultraviolet radiation sensors adjacent the ultraviolet lamp tubes to detect the presence or absence and intensity of radiation.
With the radiation sensors adjacent the ultraviolet lamps, the sensors tend to become overheated and to fail.
Sensors may also be placed adjacent the irradiated tin plate surface, usually a plane, where the ink or coating is becoming polymerized. Here too the sensors tend to become too hot and to fail.
Further, when the reflectors of the ultraviolet radiation and elliptical reflectors or substantially elliptical reflectors, with ultraviolet lamps substantially at one focus of the ellipse and the irradiated tin plate at the other focus of the ellipse, the ink to be polymerized then receives high intensity radiation in substantially lines parallel to the ultraviolet radiation producing tubes and transverse to the direction of motion of the tin plate supporting the inked message.
It is desirable to position ultraviolet detectors substantially in the focal plane of the elliptical reflectors and in the lines of highest intensity of received radiation.
Ultraviolet radiation is customarily accompanied by heat, and temperatures in the ultraviolet apparatus could reach 250.degree. F. or greater. With the focusing of radiation in a surface at the sensors the sensors are even more rapidly destroyed.